The invention relates to a method for generating measurement signals in magnetic fields which are present in the area around an NMR-MOUSE apparatus and whose changes have to be measured. Herein, a time-constant magnetic polarization field B0 is generated by magnets, for example electromagnets or permanent magnets, and a magnetic measurement field B1 is generated by means of a high frequency oscillation circuit in a pulsed manner and the echo signals S generated in the surrounding medium are measured. The echo signals can be measured by the NMR-MOUSE apparatus in a time-dependent manner after a change of the magnetic field by one or several signal impulses generated by the NMR-MOUSE apparatus in each case after an echo time tE. The measurement signal is generated in the magnetic field around the NMR-MOUSE apparatus where the components of the two magnetic field B0 and B1 extend normally to each other, that is, both fields have mutually perpendicular components.
The NMR-MOUSE apparatus (Nuclear Magnetic Resonance MObile Universal Surface Explorer) is a mobile measuring apparatus for nuclear magnetic resonance wherein the magnetic field generated and the measuring range provided by the measurement apparatus are disposed in the area around the apparatus. The NMR-MOUSE apparatus, which will simply be called xe2x80x9cNMR-MOUSExe2x80x9d, is therefore very suitable for determining data from the surrounding medium. It is used for the examination of spatial structures: It is possible to analyze therewith crystalline or glassy materials, as well as soft materials such as elastomers with regard to their molecular dynamics. Also, liquids as well as biological materials can be analyzed, see for example, G. Eidmann et al. xe2x80x9cThe NMR-MOUSE, a mobile universal surface explorerxe2x80x9d, Journal of Magnetic Resonance, 1996, pages 104/109, or A. Guthausen et al., xe2x80x9cNMR-Bildgebung und Material Forschungxe2x80x9d (NMR Imaging and material Research), Chemie in unserer Zeit, 1998, p.73/82. The time-constant static magnetic polarization field B0 is usually generated with the NMR-MOUSE by one or several permanent magnets. The pulsed magnetic measuring field B1, is the magnetic component of a high frequency field, which is formed by a coil as part of an electric oscillation circuit, wherein the coil usually serves also as the receiver coil for the echo signals to be measured. With the NMR-MOUSE, spatially homogenous magnetic fields are not necessary for the polarization of the nuclear magnetization in the permanent magnetic polarization field B0 and for the generation and detection of the measuring signals.
NMR-MOUSE apparatus can therefore be small and relatively inexpensive in comparison with normal NMR apparatus. With the NMR-MOUSE, the form and size of the surrounding volume, which is nuclear magnetically implied in the surrounding field and which is to be detected by measuring the echo signals, on one hand, the orthogonal components of both magnetic fields B0 and B1 and, on the other hand, the specific bandwidth of the magnetic excitation of the material to be examined are defined. The pattern of the magnetic field lines can be changed by the dimensioning and the arrangement of the permanent magnets and of the coil of the electric high frequency oscillation circuit.
The spatial measuring range in the surrounding medium to be examined is variable three-dimensionally by displacement of the NMR-MOUSE, by deformation of the magnetic fields by additional windings and by changing the high frequency field. It is a disadvantage of the conventional apparatus of this type that the detection of the echo signals, which are generated in the surrounding medium by the transmitter signals, is very time consuming. There is an insufficient spatial resolution within the measuring range and the signals to be measured have too little contrast for distinguishing different material properties.
As it is known from DE 195 11 835 C2, the measuring time can be shortened by scanning the measuring range with frequency-selective high frequency pulses while utilizing the given constant magnetic field. In this way, a rapid measurement value yield is obtained, however the contrast achievable is insufficient for the representation of the medium to be examined, particularly for material examinations, where the requirements are very high.
It is the object of the present invention to generate, in time-constant inhomogeneous magnetic fields, measurement signals, which make it possible to detect in the measurement field several spatial points with a problem-specific contrast in a single measuring sweep.
In a method for generating measurement signals in time constant inhomogeneous magnetic fields, which are produced by a NMR-MOUSE apparatus in the surrounding medium, a static magnetic polarization field B0 and a pulsed or oscillating magnetic polarization field B1 are generated such that echo signals S occur which are measured, and an additional pulsed magnetic field Bz is provided which affects the echo signals S so as to generate contrasts for the identification of the measuring locations where the echo signals originate.
With the additional magnetic fields, changes of the complete magnetic field in different spatial directions are obtained. If the high frequency impulses follow one another with a time spacing tEc, the additional magnetic fields are generated for the spatial coding only within the first half echo time tEc/2. The echo signals S formed thereby are subsequently called up several times with a time spacing tEc to provide thereby for contrast. In this way several spatial points can be measured in a single measuring sweep for the detection of the surrounding volume, which is called a xe2x80x9cmultiplex advantagexe2x80x9d. Furthermore, the echo signals S are re-focussed several times with the impulses of the high-frequency oscillation circuit for generating contrasts. In addition to the multiplex advantage in the surrounding space, a multiplex advantage is also provided for the contrast measurements by weighting with typical NMR parameters (for example, by transverse relaxation time). For the examination of the surrounding field to be measured therefore not only a high number of measuring points can be scanned within a measuring time unit but the measuring points are determined at the same time with increased contrast so that, by generating the pulsed additional magnetic fields, a substantial qualitative improvement is achieved with respect to the analysis with the NMR-MOUSE as compared with conventional measuring methods.
Preferably, the pulsed additional magnetic fields are time-dependent in such a way that the polarity of the additional magnetic field reverses within the echo time tEc.
For generating the high frequency signals in the pulsed magnetic measuring field B1, expediently the high frequency excitation according to the method of Carr, Purcell, Meiboon, and Gill, that is, the CPMG method, is used for the nuclear magnetization (see Guthausen et al. xe2x80x9cAnalysis of Materials by Surface NMR via MOUSExe2x80x9d, J. Magn. Reson. 130, 1998 pages 1/7). Also, other known NMR-echo procedures for contrast variation may be utilized and may be appropriate, whereby echo signals can be generated whose amplitudes are, to a large extent, independent of the inhomogeneity of an existing permanent magnetic field. The influence on the measuring signal resulting from the inhomogeneous static magnetic field as it is present in the polarization field B0 of the NMR-MOUSE is eliminated in this way. The effect of the additional magnetic field B2 on the echo signal S can therefore be determined with little interference.
A variation in the solution of the object stated above based on the method described resides in the generation of oscillation-modulated additional magnetic fields Bz(+) (for example, oscillation modulated gradient fields) and the timing tE of the high frequency impulses, and the modulation of the oscillating additional magnetic field are timed relative to each other in such a way that the time interval tE/2 between two successive high frequency impulses with rotational angles xcex1 and subsequent xcex2 (for example xcex2=2xcex1) corresponds to half the oscillation time of the modulation function of the oscillating additional magnetic field.
In this case, the two high frequency impulses are emitted as transmitter signals at the zero passage of the modulation function. During the next following zero passage of the modulation function, the high frequency impulse is omitted and, instead, the echo signal acq provided at this point in time (acq=acquisition) is detected. The high frequency impulse detection sequence xcex1xe2x88x92tE/2xe2x88x92xcex2xe2x88x92tE/2xe2x88x92acq is repeated several times wherein the additional magnetic field is amplitude-modulated after completion of each sequence. The measuring field is scanned in this way spatial point by spatial point, wherein, if necessary, missing spatial points are iteratively added for the completion of the analysis of the surrounding medium.
In order to be able to start after each sequence again at the starting point, in a refinement of the invention, the modulation function for the additional magnetic field is refocused after each detection and passage of a sequence through a half wave with an opposite, and twice the, amplitude. The k-space, or respectively, the measuring field, is interrogated in this way in a non-sequential manner for Cartesian space coordinates; from xe2x88x92k1 a jump occurs to +2 k1 and then to xe2x88x923 k1 and to 4 k1, etc. For the k1 value, the time integral of the gradient modulation function in the first time interval tE is the determinative factor. If then an iterative completion of open k-points in the measuring field is insufficient for the examination of given structure of the material, the measurement can be repeated with a sequence having a reversed sign, whereby then the space points k1, xe2x88x922 k1, +3 k1, xe2x88x924 k1, etc . . . are scanned. If the gradient modulation of the additional magnetic field occurs at constant frequency, the amplitude modulation is easily realized by a resonant control of the oscillation circuit.
Below, the invention will be described in greater detail on the basis of examples in connection with the schematic drawings.